Detrital chromian spinels from the Sa Kaeo-Chanthaburi accretionary complex, eastern Thailand : tectonic evolution of the western margin of Indochina

摘要

The geological evidence regarding the origin and tectonic setting of eastern Thailand now still be limited. In this study, I studied a melange terrane in the Sa Kaeo-Chanthaburi area, and discriminated tectono-stratigraphic units there. To establish the tectonic evolution , the geochemistry of chromian spinels is applied to determine the composition and provenance of their ultramafic-mafic rocks. Two kinds of rock assemblages are generally recognized in the Sa Kaeo-Chanthaburi area. They are a various kind of melanges, and turbidite deposits. These rock assemblages are called here as the Sa Kaeo-Chanthaburi accretionary complex. It comprises five new rock units from north to south, Khao Prik, Khao Hleam, Ban Nong Bon and Soi Dao units, and Pong Nam Ron Formation. The covering sediments, Pong Nam Ron sandstones belong to the dissected arc. In this Sa Kaeo-Chanthaburi accretionary complex, numerous detrital chromian spinels are discovered as the accessory mineral in turbiditic sandstones of the Pong Nam Ron Formation, sandstone blocks in melanges, and volcaniclastic rocks. The petrographical characteristics under the microscope indicate volcanic origins. Geochemistry the detrial chromian spinels from sandstone beds of the Pong Nam Ron Formation and sandstone blocks in melange have wider Cr content. On the other hand, detrital spinels from volcaniclastic rock have high Cr content, and vary widely in TiO2 content. The Fe3+ is consistently low in both clastic and volcaniclastic rocks. Based upon provenance discrimination fields on the Fe3+/(Cr+Al+Fe3+)-TiO2 diagram proposed by Arai (1992), plots of detrial chromian spinels in sandstones of the Pong Nam Ron Formation and sandstone blocks in melanges seem to correspond to island-arc region, whereas plots of detrital chromian spinels in volcaniclastic rock can be related with spinels from intraplate basalt and island arc basalt. ･･･